As known to those skilled in the art, linear polyesters are generally made in two stages. In the first stage, called the esterification or transesterification stage, a dicarboxylic acid or diester is reacted with a diol at elevated temperatures and at either atmospheric or elevated pressures. In this first stage water or the corresponding alcohol is produced as a byproduct. In the second stage, also known as the polycondensation stage, a vacuum is gradually applied in the presence of one or more catalysts, liberating water along with excess diol as condensation byproducts. This two-stage process is generally conducted in the melt phase, until the intrinsic viscosity of the polymer reaches about 0.2 dl/g or higher, for example, up to about 0.6 dl/g. At this point, the molten polymer is rapidly cooled to produce a solid polymer which is then pelletized or chopped. Various polyesters can be made by such polymerization techniques, including polyethylene terephthalate (PET), and various copolymers thereof.
To produce crystallizable copolymers with high molecular weights and high melting points, such as those suitable for use as bottle resins, the pelletized product of the melt phase process is subsequently subjected to solid state polymerization at a temperature below the melting point of the partially formed polymer, and in the presence of a vacuum or a nitrogen purge to remove reaction byproducts. The polymer is actually polymerized in a solid state, with the the polycondensation reaction being continued in such a state. Solid state polymerization is continued until the intrinsic viscosity of the polymer reaches any desired level, such as from about 0.6 dl/g to about 1.0 dl/g or even higher. Desirably, the intrinsic viscosity ranges from about 0.70 dl/g to about 0.90 dl/g.
Two major commercial processes are used to produce high molecular weight linear polyesters. These two processes are the ester-based process and the acid-based process, which react a diester and a diacid, respectively, with one or more diols. For example, in the production of high molecular weight polyethylene terephthalate, the dimethyl ester of terephthalic acid is heated with an excess of ethylene glycol in the presence of a transesterification catalyst at a temperature of about 185.degree. C. to about 220.degree. C. under atmospheric pressure until approximately the theoretical amount of methyl alcohol has been liberated. The excess glycol is then distilled off and the remaining product, a bis glycol ester, is polymerized by condensation. Glycol is eliminated by heating the bis glycol ester with a catalyst at elevated temperatures and under reduced pressures until a high molecular weight product is formed.
High molecular weight polyesters can also be produced on a commercial scale by an acid-based process, which is a direct esterification process. Polyethylene terephthalate, for example, can be produced by heating terephthalic acid with ethylene glycol to form a mixture of low molecular weight oligomers, which can then be polycondensed by heating in the presence of a catalyst at a temperature of about 260.degree. C. to about 300.degree. C. under reduced pressures to form a high molecular weight product. The acid-based process is currently preferred for commercial operations.
The acid-based process has many advantages, both technical and economics. The free acids are less expensive than dialkyl esters of acids. There is no lower alkyl alcohol byproduct, and since the excess of diol used is kept at a minimum, recovery and losses of diol are considerably reduced. No transesterification catalyst is required. The reaction rates are rapid and complete reaction from raw material to high polymer may be carried out in as little as three hours. Furthermore, polyesters formed by this method may attain intrinsic viscosities which are somewhat higher than those normally obtained by the ester interchange route. In addition to these advantages, the polyester product may contain less catalyst residue than polyester resin formed by the ester-based process. Although no catalyst is necessary at the initial stage of an ester based process, a catalyst such as zinc acetate, manganese acetate, or alkali metal alcoholates is typically employed as a transesterification catalyst. The only catalyst actually necessary is a condensation catalyst, which may suitably be antimony trioxide, zinc borate, litharge, lead acetate, magnesium oxide, or other condensation catalyst.
Polyester copolymers are generally prepared by combining one or more dicarboxylic acids with one or more diols, or by combining one or more diesters of dicarboxylic acids with one or more diols. A polyethylene terephthalate/naphthalate copolymer, for example, may be made by combining dimethyl terephthalate, dimethyl-2,6-naphthalene-dicarboxylate, and ethylene glycol. It is desirable to prepare such copolymers through a combination of terephthalic acid, 2,6-naphthalene dicarboxylic acid, and ethylene glycol. However, 2,6-naphthalene dicarboxylic acid, with a purity sufficient to produce high molecular weight polyester, is not currently commercially available, while its diester equivalent, dimethyl-2,6-naphthalene-dicarboxylate, is commercially available. Consequently, manufacturers that employ an ester-based process can more readily make a polyester copolymer containing both phthalate-based units and naphthalate-based units.
The conventional ester-based process to make polyethylene naphthalate (PEN) polymers employs dimethyl-2,6-naphthalene-dicarboxylate, ethylene glycol, and a catalyst, such as a manganese catalyst in the transesterification step. The presence of acidic impurities, such as the presence of terephthalic acid, would poison the catalyst, significantly reducing its activity. Thus, the presence of an acidic component would inhibit the formation of, for example, bis-(2-hydroxy-ethyl)-2,6-naphthalate, the transesterification product of dimethyl-2,6-naphthalene-dicarboxylate and ethylene glycol. It is important that all the methyl groups of dimethyl-2,6-naphthalene-dicarboxylate are completely exchanged with hydroxyethyl groups, since any residual methyl end groups will not be removed during the subsequent polycondensation reaction and will act as "dead ends" on the polymer chains, thus limiting the attainable molecular weight and rate of the melt and solid state polymerization steps.
This problem was solved by combining the product of the ester-based process with the product of the acid-based process to form a polyester copolymer as described in U.S. Pat. No. 5,594,092. More specifically, this patent describes the manufacture of a polyester phthalate/naphthalate copolymer by combining a low molecular weight naphthalate-based polymer with the acid based monomers used to make a phthalate-based polymer and/or with a phthalate-based oligomers, either at the initial stage of an acid-based polymer process, or at the second condensation stage of the acid based process.
The addition of the low molecular weight naphthalate-based polymer to the acid based process was accomplished by cooling and pelletizing the naphthalate-based polymer. To avoid making a polymer that is brittle and which shatters upon pelletizing, the reaction was carried out to make a polymer with a minimum molecular weight and intrinsic viscosity. In particular, U.S. Pat. No. 5,594,092 directs one to make a low molecular weight naphthalate-based polymer having a degree of polymerization from about 20 to 100, a number average molecular weight from about 4800 to about 24,200, and an intrinsic viscosity from about 0.15 to about 0.45 dl/g.
It would be desirable, however, to avoid manufacturing this low molecular weight naphthalate-based polymer. To make the low molecular weight naphthalate polymer, a second polycondensation stage is required whereby slightly higher temperatures are employed over the temperatures employed in the initial first stage of the reaction, vacuum is gradually applied, and a polycondensation catalyst must be added in addition to the transesterification catalyst used at the initial stage. This results in additional processing, time, equipment, and ingredients. Further, once the low molecular weight naphthalate-based polymer is added to the acid based process, the process relies upon transesterification reactions to break down the molecular weight of the naphthalate-based polymer chain and distribute the smaller chains randomly across the PET polymer as it is polymerizing. In spite of the breakdown of the polymer into smaller chain molecules, the smaller chains usually contain a number of repeated -naphthalate-glycol- bonds. It would be desirable to add an ingredient to the acid based reactants which closely approximates a reactive monomer unit in order to obtain a larger number of-phthalate-naphthalate- linkages randomly distributed throughout the polymer.